Method of producing refractory linings of centrifugal casting molds



Patented June 17, 1941 METHOD OF PRODUCING REFRACTORY LIN- IN GS OF CENTRIFUGAL CASTING MOLDS Heinrich Projahn, Gelsenkirchen, Germany, assignor to United States Pipe and Foundry Company, Burlington, N. J., a corporation of New Jersey No Drawing. Application November 16, 1938, Serial No. 240,643. In Germany February 3,

1 Claim.

For lining centrifugal casting molds fibrous materials have been used which form a felt-like coating on the inner surface of the mold and to which a binding agent may be added for preventing the washing away effect of the casting metal.

Contrary thereto I have in my prior patent application Number 240,495, filed November 15, 1938, proposed the production of centrifugal castings, especially of tubular bodies, by using as support for the casting metala mold lining which consists of loosely inserted granular materials which are held in their relative positions by selecting a sufficiently high speed of rotation so that a washing away by the metal does not take place. i

As compared with fibrous, felt-like materials the use of loosely introduced granular lining materials of approximately spherical shape presents the important advantage, that their perme ability to gas is incomparably greater than that of fibrous felt-like compositions. A high porosity of the lining is, however, of great importance in the case of centrifugal casting in order to avoid the formation of so-called pin-holes and the like in the outer surface of the casting.

The present invention relates to a particularly favourable and advantageous development of the invention according to my said prior patent application, and has for'its object, to preserve the advantages of a real granular lining mass, that is of a material which has as far as possible spherical shape, but at the same time to enable the speed of rotation to be varied within wide limits. As stated in my said prior patent application the determining critical speed of rotation depends upon the physical properties of the lining mass, especially upon the specific gravity, size of grain and shape of the constituents.

In many instances it is desirable to use substances of relatively light specific gravity, such as silver sand, magnesite and the like, but nevertheless to reduce the critical speed of rotation so as to produce the peak conditions for the casting operation. To enable the use of lower speeds of rotation without washing out of the lining by the casting metal the coherency of at least the innermost layer of the granular lining material coming directly into contact with the casting material is strengthened by addition or embedding of granular substances or bodies having a higher specific gravity or by such means which effect an addi tional hardening or binding or a solidification of the inner layer. To the granular lining materials, for example granulated iron ores, barytes or the like can be added or embedded therein. Another possibility is to add small quantities of a binding agent to the granular lining mass or only to the innermost layer thereof before introduction into the mold or to apply this subsequently onto the innermost layer which is sufficient to allow the desired reduction in speed but causes no detrimental gas generation during the casting and does not effect the gas-permeability of the lining contrary to the known shell-like cement coatings impermeable to gas.

Whereas, hitherto in moist masses to be molded containing clay and binding agents the surface of the composition has been merely smoothed with a bar fixed on a tipping trough, the cohesion of the granular lining materials may also be increased for the purpose of reducing the critical speed, by applying additional pressure onto the mold lining from the inner side, for example by profiled rolls or the like. The effect is further increased by the simultaneous use of a suitable binding agent and heating of the lining, the composition can either be introduced in hot state or be heated after introduction by internal heating. The internal heating of a centrifugal mold lining is known, but only with the object of driving the moisture out of green sand molds for the purpose of drying and in another instance for producing a smooth fused layer on the inner surface of the lining. Contrary thereto the application of heat in the invention working as far as possible with dry, granular lining materials, serves solely for the purpose of assisting the cohesion of the granular'constituents, that is of exerting so to say a sintering action in order to reduce the speed which would be necessary without increase of the cohesion of the granular constituents.

Another possibility of effecting the desired solidification of the mold lining consists in introducing the granular lining materials not with a tipping trough but by means of a trough and, if desired, under gas pressure. It has been found that the constituents of the composition then pack more tightly and-are secured in their position also at lower speed than when the composition is introduced with a tipping trough or the like.

Experiments have shown that a reduction of the critical speed of rotation and an increase in the cohesion of the innermost layer can be attained by applying'on the finished lining composed of granular materials from the interior a thin layer of different materials like a facing. A fine graphits blackmay, for example, be applied as facing.

It is also possible to use this facing layer mixed with a dry or even liquid bending agent. It is particularly advisable, to use emulsions or colloidal solutions of such materials in water-glass or even in oleaginous binding agents or to incorporate or apply boric acid or similar substances behaving like glass under not too great heat, in or on the innermost layer coming into contact with the casting material. Metallic substances, such as aluminium powder or even metal oxides, such as aluminium oxide, magnesia can also be applied alone or with a suitable binding agent. The use of such facings presents the advantage that they fuse on to the casting in varying thicknesses and facilitate the drawing of the casting.

It is also possible, to use in known manner a wire fabric or the like and to arrange this on or in the inner surface of the mold lining composed of granular materials, with the result that a reduction of the critical speed is also rendered possible.

As already briefly mentioned, it is known to useas lining compositions for centrifugal molds substances which are known as mully" or slurry and consist chiefly of clay, loam, sand and considerable quantities of water. Such mold compositions, contrary to those used for the production of thelining according to the invention, require very intensive drying and, owing to the high percentage of clay and loam, are only slightly permeable to gas, but furthermore, owing to their clay and loam contents, they become hard under the action of the casting heat so that they can only be destroyed by mechanical means, and the castings can only be drawn after the destruction of the composition casing enclosing the same. It is important for the case in question that the agent assisting the cohesion of the inner layer coming into direct contact with the casting material, does not reduce the porosity and trickling property of the granular lining composition and that materials are used as lining layer which preferably have only point contact, and that the percentage of moisture, when using binding agents, is kept so low that both intensive drying and also considerable gas generation are avoided during the casting.

I claim:

A process for the production of refractory linings of rotary molds for use in centrifugal casting consisting in introducing into a mold casing loose, dry, granular, refractory material, forming a granular lining by rotating said mold casing to distribute said material and to hold the same in place by centrifugal force only, and forming at the inner surface of the granular lining, during the rotation thereof, a zone having greater resistance than the rest of the lining to the washing-away effect of the metal to be cast, said zone being formed by applying a binding agent to produce increased cohesion between the particles of the innermost layer of the granular lining.

HEINRICH PROJAHN. 

